Why did you put in -r. Why was there an r at all. If r = 5, you're saying 25 ≠ -5. Well yh, but this will happen to every number besides -1 and 0. If r = -1, you're saying 1≠1, which can't be true.
Instead of r^2 = -r, you could have r^2 = -m where m is a real positive number. but the problem with this is that it won't include 0. So the only way is to do r^2 ≥ 0